Recent years have seen the development of projectors having configurations that use a semiconductor light-emitting element such as a laser diode or LED (Light-Emitting Diode) as a light source for the purpose of realizing longer life of the light source and lower power consumption. In such projectors, the LED or laser diode is normally capable of supplying only light of a single wavelength, and configurations are thus known that use a phosphor to generate light (colored light) of a wavelength that differs from the light that is generated in the light source.
FIG. 1 is a block diagram showing an example of the configuration of a projector provided with a phosphor. FIG. 2 shows an example of the configuration of the phosphor wheel shown in FIG. 1, (a) of this figure being a plan view of the phosphor and (b) being a plan view of the phosphor wheel.
FIG. 3 is a graph showing an example of the quantity of emitted light of the phosphor wheel shown in FIG. 1, FIG. 4 is a graph showing an example of the quantity of emitted light of two phosphor wheels having the characteristics shown in FIG. 3 and the quantity of the synthesized light of these phosphor wheels.
The projector shown in FIG. 1 is provided with excitation light source 101, dichroic mirror 102, phosphor wheel 103, display device 104, projection lens 105, video processing circuit 106, phosphor rotation control unit 107, motor drive circuit 108, and motor 109.
Light that is emitted at excitation light source 101 is reflected by dichroic mirror 102 and irradiated upon the phosphor that is fixed upon phosphor wheel 103. The phosphor is excited by the light from excitation light source 101 and emits light of a wavelength that differs from this light. Dichroic mirror 102 is formed to have the characteristics of reflecting light that is emitted by excitation light source 101 and transmitting light that is emitted by the phosphor. As a result, light that is emitted by the phosphor is transmitted through dichroic mirror 102 and irradiated upon display device 104. Display device 104, by optically modulating the irradiated light in accordance with a picture signal that is supplied from video processing circuit 106, forms a picture that corresponds to the color of the irradiated light. The picture formed by display device 104 is enlarged and projected upon, for example, a screen (not shown) by projection lens 105.
Video processing circuit 106 both supplies a picture signal to display device 104 and supplies a signal indicating the drive cycle of display device 104 to phosphor rotation control unit 107. Phosphor rotation control unit 107 realizes control such that the rotational speed of motor 109 is fixed by means of motor drive circuit 108 in accordance with a signal that indicates the drive cycle of display device 104. Phosphor wheel 103 is secured to the rotational axis of motor 109, and phosphor wheel 103 rotates while synchronized to, for example, a signal indicating the drive cycle of display device 104 that is supplied from video processing circuit 106.
Phosphor wheel 103 is of a configuration in which phosphor 201 that is thinly cut as shown in FIG. 2 is adhered and fixed upon disk 202. When this phosphor wheel 103 is rotated by means of motor 109, light from excitation light source 101 is not continuously irradiated upon one point of phosphor 201. As a result, damage by burning of phosphor 201 by light from excitation light source 101 can be prevented. In addition, phosphor 201 is further cooled by the rotation of phosphor wheel 103 and phosphor 201 can therefore be caused to emit light efficiently.
Nevertheless, as shown in FIG. 2 (a) and (b), phosphor 201 is adhered and fixed upon disk 202 along the circumference divided into, for example, six segments, and therefore has seams 203 between adjacent phosphors 201. Accordingly, light that is emitted by phosphor wheel 103 cyclically decreases in brightness (quantity of emitted light) as shown in the graph of FIG. 3. The cycle of this decrease in the quantity of emitted light is synchronized with the drive cycle of display device 104 shown in FIG. 1 and therefore causes flicker in the projected picture.
Still further, in order to achieve an increase in the brightness of projected light, projectors in recent years may be of a configuration provided with a plurality of phosphor wheels 103 in which light emitted by each phosphor wheel 103 is synthesized and the light that follows synthesis is irradiated upon display device 104. In a configuration provided with such a plurality of phosphor wheels 103, this flicker in the projected picture may be exacerbated.
For example, in a projector provided with two phosphor wheels, one phosphor wheel is “a” and the other phosphor wheel is “b”. A case will be considered in which, as shown in FIG. 4, the quantity of emitted light of portions of phosphor wheel a other than seams 203 is La, the quantity of emitted light that decreases at seams 203 is ΔLa, the quantity of emitted light of portions of phosphor wheel b other than seams 203 is Lb, the quantity of emitted light that decreases at seams 203 is ΔLb, and phosphor wheels a and b are rotated in synchronization.
In this case, as shown by the quantity of synthesized light of FIG. 4, when phosphor wheels a and b rotate such that seams 203 coincide (are synchronized), the quantity of emitted light following synthesis realized by the two phosphor wheels a and b is La+Lb, but the quantity of the decrease in emitted light at seams 203 increases to ΔLa+ΔLb. As a result, flicker in the projected picture increases.
Configurations in which a semiconductor light-emitting element is used as the light source and light of a wavelength that differs from the light emitted at the light source is generated by using a phosphor are described in Patent Documents 1 and 2.
Patent Document 1 discloses an improvement in the chromaticity of the blue light that irradiates a display device by providing a laser diode that emits laser light of a blue wavelength band and a phosphor that, using the laser light of the blue wavelength band as excitation light, emits light of a blue wavelength band that differs slightly in wavelength from the laser light of the blue wavelength band and then synthesizes the laser light of the blue wavelength band and the light of the blue wavelength band that is emitted by the phosphor.
Patent Document 2 discloses the adjustment of the quantity of emitted laser light of a blue wavelength band by a laser diode by performing PWM (Pulse Width Modulation) modulation of the current that is supplied to the laser diode that emits laser light of the blue wavelength band.